We need to know all sorts of details: size of tank and how many fish, how long has the tank been set up, do you test the water and how much/ how often do you change water. It would also be helpful to know if he can right himself at all, if he has ever had issues with flipping or floating before and what his diet is like. I hesitate to say swim bladder disorder without more info. Try doing a water change and feed him only peas for awhile (steam them, shell/peel them and cut into bite-size pieces)

I had a fish that would do this in quarantine after buying, the poor oranda had eaten nothing but flakes for several days, so when I started keeping him on a diet of tubifex and peas he was getting better

Other than that, we need to know your tank size and maintenance schedule, like cheri said